Meiosis

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This presentation is with consent from Mrs. Janice Williamson

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    Meiosis Meiosis Presentation Transcript

    •  In humans, somatic cells (body cells) have:• 23 pairs of homologous chromosomes and• one member of each pair from each parent. The human sex chromosomes (Gonosomes)X and Y differ in size and genetic composition. The other 22 pairs of chromosomes areautosomes with the same size and geneticcomposition.UNIT 3: MEIOSISREDUCTION DIVISION1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS© 2012 Pearson Education, Inc.
    •  Homologous chromosomes are matched in:• length,• centromere position, and• gene locations (locus). A locus (plural, loci) is the position of a gene. Different versions (alleles) of a gene may befound at the same locus on maternal andpaternal chromosomes.© 2012 Pearson Education, Inc.Centromere
    •  Homologous chromosome pair© 2012 Pearson Education, Inc.Centromere
    •  Humans and most animals and plants have diploidbody cells. That means they have two sets of chromosomes(homologous chromosome pair) one from eachparent. Diploid is written 2n. It refers to the total number of chromosomesa cell can have.2. GAMETES HAVE A SINGLE SET OFCHROMOSOMES© 2012 Pearson Education, Inc.
    •  Meiosis is a process that converts diploidnuclei to haploid nuclei.• Diploid cells have 2 sets of chromosomes.• Haploid cells have 1 set of chromosomes.• Meiosis occurs in the sex organs,producing gametes—sperm and eggs. Fertilization is the fusion of a sperm and eggcell. The zygote has a diploid chromosomenumber, one set from each parent.© 2012 Pearson Education, Inc.
    • Haploid gametes (n  23)Egg cellSperm cellFertilizationnnMeiosisOvary TestisDiploidzygote(2n  46)2nMitosisKeyHaploid stage (n)Diploid stage (2n)Multicellular diploidadults (2n  46)A life cycle
    •  All sexual life cycles include an alternationbetween• a diploid stage and• a haploid stage. Why is meiosis so important? It produceshaploid gametes which prevents thechromosome number from doubling in everygeneration. Produce gametes for fertilization.© 2012 Pearson Education, Inc.
    • 3Meiosis is a type of cell division thatproduces haploid gametes from diploidcells.Two haploid gametes combine infertilization to restore the diploid state inthe zygote.3. MEIOSIS
    • 3SUMMERY OF THE MEIOSIS PROCESS
    • MEIOSIS I consisting of 5 phases:Interphase I, Prophase I, Metaphase I,Anaphase I, Telophase I.MEIOSIS II consisting of 4 phasesProphase II, Metaphase II, Anaphase II,Telophase II.MEIOSIS HAS 2 STAGES:© 2012 Pearson Education, Inc.
    • Cell build up energyDNA Replication (to makeduplicated chromosomesCell doesn’t changestructurally.MEIOSIS I : INTERPHASE© 2012 Pearson Education, Inc.
    •  Events occurring in the nucleus:• Chromosomes coil and become individual chromo-somes, nucleolus and nuclear envelope disappear.• Homologous chromosomes come together as pairs bysynapsis forming a tetrad (Each pair, with fourchromatids)• Non-sister chromatids exchange genetic material throughthe process of crossing over to ensure geneticvariation.• Centrioli move to opposite poles with spindle fibersbetween them.MEIOSIS I : PROPHASE I
    • MEIOSIS I : PROPHASE I© 2012 Pearson Education, Inc.
    •  Genetic recombination is the production of newcombinations of genes due to crossing over. Crossing over is an exchange of genesbetweenseparate (non-sister) chromatids on homologouschromosomes.• Non-sister chromatids join at a chiasma(plural, chiasmata), the site of attachment.• Genetic material are exchanged betweenmaternal and paternal (nonsister) chromatids.CROSSING OVER© 2012 Pearson Education, Inc.
    • CROSSING OVER© 2012 Pearson Education, Inc.
    •  Centrioli has reached thepoles. Homologous pairs align atthe cell equator. The two chromosomes attachto one spindle fiber by meansof the kinetochore of thecentromere..MEIOSIS I: METAPHASE I© 2012 Pearson Education, Inc.
    •  Spindle fibers contract. Duplicated chromosomesmove to opposite poles..MEIOSIS I: ANAPHASE I© 2012 Pearson Education, Inc.
    • • Duplicatedchromosomes havereached the poles.• A nuclear envelope andnucleolus re-formsaround chromosomes.• Each nucleus now hasthe haploid number ofchromosomes.• Cell invaginates forminga cleavage furrow, whichextends to for 2 separatehaploid cells.MEIOSIS I: TELOPHASE I© 2012 Pearson Education, Inc.
    •  Follows meiosis I without chromosomeduplication. Each of the two haploid products entersmeiosis II.MEIOSIS II© 2012 Pearson Education, Inc.
    • • Chromosomes coil andbecome compact (ifuncoiled after telophase I).• Nuclear envelope andnucleolus, if re-formed,dissappears again.• Centrioli move to oppositepoles, forming spindlefibers between them.MEIOSIS II: PROPHASE II© 2012 Pearson Education, Inc.
    • • Individual duplicatedchromosomes align on theequator.• One chromosome per spindlefiber attached by means ofkinetochore of centromere.• Centrioli has reached thepoles.MEIOSIS II: METAPHASE II© 2012 Pearson Education, Inc.
    • • Spindle fibers contract.• Duplicated chromosomessplit in half (centromeredividing in 2)• Daughter chromosomesmove to opposite poles.MEIOSIS II: ANAPHASE II© 2012 Pearson Education, Inc.
    • • Daughter chromosomes hasreached the poles.• Two cells invaginate and form 4daughter haploid cells(gametes)• They uncoil and formchromatin.• Nuclear envelope andnucleolus for around chromatinagain.• Centrioli for centrosome.MEIOSIS II: TELOPHASE II
    • SUMMERY OF MEIOSIS IIProphase II Metaphase II Anaphase IIHaploid daughtercells formingTelophase IIand Cytokinesis
    •  Mitosis and meiosis both• begin with diploid parent cells that• have chromosomes duplicated during theprevious interphase. However the end products differ.• Mitosis produces two genetically identicaldiploid somatic daughter cells.• Meiosis produces four genetically uniquehaploid gametes.4. SIMILARITIES AND DIFFERENCES BETWEENMITOSIS AND MEIOSIS
    • • Independent orientation at metaphaseI• Random fertilization.• Crossing over of genes duringprophase I5. GENETIC VARIATION IN GAMETESRESULTS FROM:© 2012 Pearson Education, Inc.
    • 6. KARYOTYPE© 2012 Pearson Education, Inc.• A karyotype is an ordered display ofmagnified images of an individual’schromosomes arranged in pairs.• Karyotypes allow for the observation of : homologous chromosome pairs, chromosome number, and chromosome structure.
    • SCIENTIST OBSERVING A HUMANKARYOTYPE
    • CentromereSisterchromatidsPair ofhomologouschromosomesSex chromosomes
    •  An extra copy of chromosome 21 causesDown syndrome or also known as TRISOMY21. A. Trisomy 21• involves the inheritance of three copies ofchromosome 21 and• is the most common human chromosomeabnormality.7. ALTERATION IN CHROMOSOME NUMBER© 2012 Pearson Education, Inc.
    • Down syndrome
    •  Trisomy 21 produces a characteristic set of symptoms,which include:• mental retardation,• characteristic facial features,• short stature,• heart defects,• susceptibility to respiratory infections, leukemia,and Alzheimer’s disease, and• shortened life span. The incidence increases with the age of the mother.
    •  Nondisjunction is the failure of chromosomes orchromatids to separate normally during meiosis. Thiscan happen during:• meiosis I, if both members of a homologous pair goto one pole or• meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes withaltered numbers of chromosomes.B. ACCIDENTS DURING MEIOSIS CANALTER CHROMOSOME NUMBER
    • NondisjunctionMEIOSIS IMEIOSIS IINormalmeiosis IIGametesNumber ofchromosomesAbnormal gametesn  1 n  1 n  1 n  1
    • Normalmeiosis IMEIOSIS IMEIOSIS IINondisjunctionAbnormal gametes Normal gametesn  1 n  1 n n
    • Sex chromosome abnormalities tend tobe less severe, perhaps because of• the small size of the Y chromosomeor• X-chromosome inactivation.C. ABNORMAL NUMBERS OF SEXCHROMOSOMES© 2012 Pearson Education, Inc.
    •  In general,• a single Y chromosome is enough to produce―maleness,‖ even in combination with severalX chromosomes, and• the absence of a Y chromosome yields―femaleness.‖© 2012 Pearson Education, Inc.
    •  The following table lists the most common human sexchromosome abnormalities.© 2012 Pearson Education, Inc.
    •  Errors in mitosis or meiosis may producepolyploid species, with more than twochromosome sets. .D. NEW SPECIES CAN ARISE FROMERRORS IN CELL DIVISION© 2012 Pearson Education, Inc.
    •  Chromosome breakage can lead torearrangements that can produce:• genetic disorders or,• if changes occur in somatic cells, cancer.8. ALTERATIONS OF CHROMOSOMESTRUCTURE© 2012 Pearson Education, Inc.
    • • a deletion, the loss of a chromosomesegment,• a duplication, the repeat of a chromosomesegment,• an inversion, the reversal of a chromosomesegment, or• a translocation, the attachment of a segmentto a nonhomologous chromosome that can bereciprocal.THESE REARRANGEMENTS MAY INCLUDE:
    • THESE REARRANGEMENTS MAY INCLUDE:© 2012 Pearson Education, Inc.DeletionDuplicationInversionReciprocal translocationHomologouschromosomes Nonhomologouschromosomes
    • • 1ste: Some students might conclude that sex chromosomes function only in determiningthe sex of the individual. As the authors note, sex chromosomes contain genes notinvolved in sex determination. Students might recall some basic genetics, rememberingthat for many traits a person receives a separate ―signal‖ from mom and dad. Theseseparate signals for the same trait are carried on the same portion of homologouschromosomes, such as the freckle trait noted in Module 8.11.• 4de: Student Students might not immediately see the need for meiosis in sexualreproduction. Consider an example of what would happen over manygenerations if gametes were produced by mitosis. The resulting genetic doublingis prevented if each gamete has only half the genetic material of the adult cells.1.Consider helping students through mitosis and meiosis by developing an analogyto pairs of shoes. In this case, any given species has a certain number of pairs ofshoes, or homologous chromosomes. 2. In the shoe analogy, females have 23pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . .maybe a sandal and a sneaker! 3. You might want to get your students thinkingby asking them why eggs and sperm are different. (This depends upon thespecies, but within vertebrates, eggs and sperm are specialized for differenttasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain anucleus and most of the cytoplasm of the future zygote. Thus eggs are typicallylarger, nonmotile, and full of cellular resources to sustain cell division andgrowth.)
    • • 10de: Students might not immediately see the need for meiosis in sexual reproduction. Consideran example of what would happen over many generations if gametes were produced by mitosis.The resulting genetic doubling is prevented if each gamete has only half the genetic material ofthe adult cells. Challenge students to identify which stage of meiosis is most like mitosis.Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows studentsto identify essential differences.• 14de: 1. If you wish to continue the shoe analogy, crossing over is somewhat like exchangingthe shoelaces in a pair of shoes (although this analogy is quite limited). A point to make is thatthe shoes (chromosomes) before crossing over are what you inherited . . . either from the spermor the egg; but, as a result of crossing over, you no longer pass along exactly what youinherited. Instead, you pass along a combination of homologous chromosomes (think of shoeswith switched shoelaces). Critiquing this limited analogy may also help students to think throughthe process of crossing over. 2. In the shoe analogy, after exchanging shoelaces, we have―recombinant shoes‖! 3. Challenge students to consider the number of unique humans that canbe formed by the processes of the independent orientation of chromosomes, randomfertilization, and crossing over. Without crossing over, we already calculated over 70 trillionpossibilities. But as the text notes in Module 8.17, there are typically one to three crossoverevents for each human chromosome, and these can occur at many different places along thelength of the chromosome. The potential number of combinations far exceeds any number thathumans can comprehend, representing the truly unique nature of each human being (animportant point that delights many students!)
    • • 26ste: 1. The possible number of combinations produced by independentorientation of human chromosomes at meiosis metaphase I is 223 or8,388,608. This number squared is more than 70 trillion. The authorsrounded down to 8 million for 223 and squared this, to estimate 64 trillionpossible combinations. But more precisely, the number of possiblezygotes produced by a single pair of reproducing humans, based solelyon independent assortment and random fertilization, is over 70 trillion! 2.Another way to represent the various combinations produced byindependent orientation of chromosomes at meiosis metaphase Icontinues the shoe analogy. Imagine that you have two pairs of shoes.One pair is black, the other is white. You want to make a new pairdrawing one shoe from each original pair. Four possible pairs can bemade. You can have (1) the left black and left white, (2) the right blackand right white, (3) the left black and right white, or (4) the right blackand left white. Actually using two pairs of shoes from your students caninject humor and create a concrete example that reduces confusion. Foran additional bit of humor, ask the class if 46 students want to contributetheir shoes as you try to demonstrate all 8,388,608 combinations!
    • 14de: 1. If you wish to continue the shoe analogy, crossing over is somewhatlike exchanging the shoelaces in a pair of shoes (although this analogy is quitelimited). A point to make is that the shoes (chromosomes) before crossing overare what you inherited . . . either from the sperm or the egg; but, as a result ofcrossing over, you no longer pass along exactly what you inherited. Instead,you pass along a combination of homologous chromosomes (think of shoeswith switched shoelaces). Critiquing this limited analogy may also helpstudents to think through the process of crossing over. 2. In the shoe analogy,after exchanging shoelaces, we have ―recombinant shoes‖! 3. Challengestudents to consider the number of unique humans that can be formed by theprocesses of the independent orientation of chromosomes, randomfertilization, and crossing over. Without crossing over, we already calculatedover 70 trillion possibilities. But as the text notes in Module 8.17, there aretypically one to three crossover events for each human chromosome, andthese can occur at many different places along the length of the chromosome.The potential number of combinations far exceeds any number that humanscan comprehend, representing the truly unique nature of each human being(an important point that delights many students!)
    • • 26ste: 1. The possible number of combinations produced by independentorientation of human chromosomes at meiosis metaphase I is 223 or8,388,608. This number squared is more than 70 trillion. The authorsrounded down to 8 million for 223 and squared this, to estimate 64 trillionpossible combinations. But more precisely, the number of possible zygotesproduced by a single pair of reproducing humans, based solely onindependent assortment and random fertilization, is over 70 trillion! 2.Another way to represent the various combinations produced byindependent orientation of chromosomes at meiosis metaphase I continuesthe shoe analogy. Imagine that you have two pairs of shoes. One pair isblack, the other is white. You want to make a new pair drawing one shoefrom each original pair. Four possible pairs can be made. You can have (1)the left black and left white, (2) the right black and right white, (3) the leftblack and right white, or (4) the right black and left white. Actually using twopairs of shoes from your students can inject humor and create a concreteexample that reduces confusion. For an additional bit of humor, ask theclass if 46 students want to contribute their shoes as you try to demonstrateall 8,388,608 combinations!
    • • 30ste: Before addressing karyotyping and nondisjunction events,consider reviewing the general structure and terminologyassociated with replicated chromosomes and the arrangement ofchromosomes during metaphase of mitosis, meiosis I, andmeiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firmfoundation in chromosome basics is necessary to understand theirregularities discussed in Modules 8.19–8. 1. The HumanGenome Website is a tremendous asset for nearly everydiscussion related to human genetics. It can be accessed atwww.genomics.energy.gov. 2. If you have several hundredstudents or more in your class, it is likely that at least one of yourstudents has a sibling with Down syndrome. The authors notethat, overall, about one in every 700 babies are born with Downsyndrome. 3. The National Down Syndrome Society has awebsite at www.ndss.org. It is a wonderful resource.